Electric power cables in the UK

general internal cable - three plus earth - armoured cable

Domestic electric cable is usually available to British Standard BS 6004.

31st March 2004 saw the introduction of a new colour code (harmonised across the EU) for power cables (See this page for details of these changes). Until 31st March 2006, either the old colours or new colours may be used (but not mixed). Below we give both the old and new cable colours.

The standard UK domestic electricity supply is 230 volts AC.

Solid core cables should never be reused - although they can be bent into shape, they are not designed to be flexed and repeated movement can weaken the cores causing then to become weak, overheat or fail.

Twin core and earth

(general internal power cables)

Consists of two insulated solid cores with a bare copper earth conductor between the inner cores and an overall PVC sheath. The colours being:

Note: the earth feed is normally bare copper in mains installation cable, the Green and Yellow applies to the coloured sleeving which needs to be fitted where the earth is connected to a fitting or appliance.

Note: The currents and wattages shown are for the cables in 'free air' installations, the cables should be derated if cables are run together or in such a way that the thermal rise of the cable is not free to dissipate.

Types of cables that may be found in older premises.

Rubber insulated, two core (multi strand) covered with an outer lead sheath. If there is any of this very old wiring still installed, it should be isolated and replaced immediately. With the age of this type of cable, it is almost guaranteed that the rubber insulation will be degraded and breaking down, any failure of the insulation can cause the outer covering to become live - an installation still using this type of wiring is unlikely to have modern trips fitted in the consumer unit or a master earth leakage trip.

Rubber insulated, two or three core (multi strand) covered with an outer rubber sheath. The danger with this type of cables is the degrading of the rubber especially where a terminal has become overheated. The wiring should be considered ready for replacement.

PVC insulated, three core (multi strand) covered with an outer PVC sheath, this may still be in good physical condition. Two points to watch for are:

1. Broken individual strands - this will reduce the current carrying capacity of the cable and can cause the wire to overheat at screw terminals.
2. Earth wires which are just twisted together - this is not a function of the cable but the method of installation common at the time this type of cable was available.

3-core and earth

(normally used for the connections between two way switches)

Consists of three insulated solid cores with a bare copper earth conductor between the inner cores and an overall PVC sheath. The colour codes are:

Armoured cable

Armoured cable is normally used to transfer power either underground or overhead. The construction is normally the inner cores (which are generally multi strand) being individually sheathed, then covered by an initial overall plastic sheath, followed by the protective wire armour with a final outer sheath to hold it all together. The inner cores may number two, three or four depending upon the application.

The wire protective armour should always be earthed at at the supply end to ensure that it cannot become live if the the cable is severed or a fault should develop within the cable.

Installation of armour cable should be left to professional contractors, not so much because it is difficult but because of the safety implications if overhead or underground installations should fail to meet the exacting installation requirements.

The ratings below are typical for three core armoured cable but individual manufacturers will give specific ratings for their own cables which should be used in preference to the following.

general internal cable - three plus earth - armoured cable